If the entire rig was floating, you could grab onto either end and pull it out. If it was easier to grab the trailer end, they probably did and began to pull it ashore. This picture may have been taken after the tow has started and the trailer was out of the water. This would kinda make sense for the time to take it since the truck is now coming out of the water.
However, since the window appears to be down in the truck I'm thinking that the boyancy just wouldn't be there for more than a minute or two, to keep the truck floating, if that, and certainly not by the time the tow truck go there, even if the windows on the truck were up.
My bet is that the owner approached the ramp front ways to talk to someone he knew, or to check the water before going to the staging area and did not set the parking brake correctly, or it failed after he had exited the truck, and gravity took care of the rest.
Chris